The protozoan parasite Entamoeba histolytica causes an estimated 50 million cases of invasive disease annually. The most common manifestation of amebic infection is colonic disease (dysentery). Several virulence factors have been identified that are important for parasite pathogenesis, however the molecular mechanisms that the parasite utilizes as it establishes invasive disease remain unclear. Transcriptional regulation of amebic genes has been shown in a number of biologically relevant conditions including encystation, phagocytosis, drug resistance, stress and nutrient deprivation. Our hypothesis is that amebic virulence determinants that are important in causing invasive colonic disease will be regulated by the stimuli present in the colonic environment and can be identified by methods that detect message abundance. Our goal is to develop and use E. histolytica DNA micro-arrays to investigate amebic colonic pathogenesis. To identify virulence determinants that are important in causing colonic disease we will identify amebic genes that are regulated due to interaction of the parasite with (i) enteric bacteria, (ii) colonic epithelial cells, and (iii) erythrocytes. These conditions to a large extent mimic the environment encountered by the parasite within the host colon. For each given condition we will identify cohorts of genes whose transcript abundance is increased, decreased, or unchanged. Ultimately we would hope to identify genes, which are coordinately regulated (i.e. induced under all the conditions tested or repressed under all the conditions tested) as these may represent virulence determinants in causing colonic disease. Genes of interest will be characterized and their roles in amebic invasiveness and virulence investigated using molecular and genetic approaches. This approach promises to unveil novel molecular aspects of amebic pathogenicity and provide a number of genetic targets for improved diagnostic and treatment strategies. Additionally, the micro-arrays generated as a part of this grant will have a multitude of uses and will prove useful to a number of scientists in the scientists in the Entamoeba scientific community to study varied aspects of amebic pathogenesis.